In 2010, Fonseca was cast in a starring role in the CW television series Nikita as Alex, a new recruit,[7] a role she played throughout the series' four seasons. During its run, the Nikita cast included her Kick-Ass co-star Xander Berkeley and her future husband, co-star Noah Bean.

Fonseca married Matthew Smiley in April 2009. The couple separated in July 2012 and Fonseca filed for divorce in 2013, citing "irreconcilable differences".[8]

In February 2016, Fonseca announced her engagement to her Nikita co-star, actor Noah Bean.[9] She and Bean married on October 2, 2016 in Connecticut.[10][11] Fonseca and Bean have a daughter, Greta, born in February 2018.[12][13]